Among biochemical reactions, the reaction of binding a receptor and a ligand is important. For example, in an antigen-antibody reaction, an antibody as a receptor binds to an antigen as a ligand. For example, in an enzymatic reaction, an enzyme as a receptor binds to an enzyme substrate as a ligand. By utilizing these reactions, a ligand present in a sample can be detected.
For example, a receptor is bound to a metal thin film surface. When a ligand binds to the receptor, the dielectric constant in the vicinity of the metal thin film is changed. A method of using a surface plasmon resonance phenomenon to detect such a change and thereby analyze the amount of a receptor-ligand complex, has been known (see, for example, Non-patent document 1). In this method, initially, a metal thin film to whose surface a receptor is bound as described above is irradiated from the rear side with light at an angle that satisfies a total internal reflection condition. When the angle is a specific incident angle that causes the wave number of a surface plasmon excited by the incident light to be equal to the wave number of an evanescent wave derived from the excitation light, a portion of the amount of the incident light is used for excitation of the surface plasmon, so that the amount of reflected light is reduced. For example, in order to detect the dielectric constant change in the vicinity of the metal thin film, a method of measuring reflected light while changing the angle of incident light to determine an angle of incident light at which absorption is highest, and a method of determining an angle of reflected light at which absorption is highest where the angle of incident light is held constant, have been known. In addition, a technique of applying an electric field (electrical vibration) to the rear side of a metal thin film on which a receptor is immobilized to control separation and movement of a sample on the metal thin film (see, for example, Patent document 1) and a technique of using a change in refractive index of a metal thin film to measure a large amount of sample at a time (see, for example, Patent document 2), have been proposed.
However, in these methods, the amount of a ligand binding to a receptor is analyzed based on a minute amount of angle change corresponding to a reduction in light amount due to excitation of surface plasmon. Therefore, in these methods, the surface of a metal thin film (e.g., thickness: about several nanometers) on which incident light is reflected needs to be even. In addition, since a minute angle change amount is measured based on a change in light amount, optical parts need to be adjusted with high precision so that light can be received without deviation of an optical axis. As a result, apparatuses for use in these methods tend to be influenced by optical design. In addition, since a minute change is detected, reading precision deteriorates significantly when an apparatus suffers from vibration during measurement. Therefore, it is difficult to produce portable apparatuses that are suitable for these methods.    Non-patent document 1: Anal. Chem., 1998, 70,2019-2024    Patent document 1: JP 2003-65947 A    Patent document 2: JP 2003-75336 A